The Optical Transport Network (OTN) has been recommended by International Telecommunication Union-Telecommunication Standardization Sector (ITU-T). The OTN employs wavelength division multiplexing (WDM) that can cope with an explosive increase in the Internet traffic and functions as a platform that makes lower layers transparent to upper layers in transmitting a client signal end to end. The OTN may be applied to synchronous networks such as the Synchronous Digital Hierarchy (SDH) and the Synchronous Optical Network (SONET) as well as to asynchronous networks such as an Internet protocol (IP) network and the Ethernet (registered trademark). Interfaces and frame formats of the OTN have been standardized by the ITU-T recommendation G.709 and the application of the OTN to commercial systems has been rapidly increasing.
Multiplexing and demultiplexing of signal transmission frames (may be called lower-speed signal transmission frames) such as optical channel data units “j” (ODUj) with a lower signal rate (e.g., bit rate) and signal transmission frames (may be called higher-speed signal transmission frames) such as optical channel data units “k” (ODUk) with a signal rate (e.g., bit rate) higher than that of the ODUj in a network employing interfaces conforming to the ITU-T recommendation G. 709 are discussed below.
Here, an ODU frame containing a client signal of, for example, the Ethernet (registered trademark) is called a lower order ODU (LO_ODU) and an ODU frame containing multiple lower-speed ODU frames is called a higher order ODU (HO_ODU). Accordingly, multiple lower-speed ODUj (e.g., ODU1) frames are multiplexed in a higher-speed HO_ODUk frame (e.g., ODU2, ODU3, or ODU4). Lower-speed ODUj frames are not limited to LO_ODUj frames. That is, HO_ODUj frames may also be multiplexed in an HO_ODUk frame.
FIG. 1 illustrates an OTUk frame format. An OTN frame may include an overhead, an optical channel payload unit k (OPUk, k is zero or a positive integer), and an optical channel transport unit k forward error correction (OTUk FEC).
The overhead is composed of first through sixteenth columns and four rows and has a size of 16 bytes×4. The overhead includes a frame alignment (FA) overhead, an OTUk overhead, an ODUk overhead, and an OPUk overhead, and is used for connection and quality management. The OPUk payload is composed of 17th through 3824th columns and 4 rows, and has a size of 3824 bytes×4. The OTUk FEC is composed of 3825th through 4080th columns and 4 rows, has a size of 256 bytes×4, and is used to correct an error caused during transmission.
The FA overhead includes a frame alignment signal (FAS) that is a fixed frame pattern of six bytes and a multiframe alignment signal (MFAS) that is a sequence number of one byte.
When ODUj frames are multiplexed in an HO_ODUk frame, an OPUk payload area of the HO_ODUk frame is divided into ts time slots called tributary slots (TS) in units of bytes, and an ODUj frame (s) is placed in each TS of the OPUk payload area.
In the ITU-T recommendation G.709, two types of tributary slots with different bit rates (or granularities) are defined: a tributary slot with a bit rate of about 1.25 Gbps and a tributary slot with a bit rate of about 2.5 Gbps (hereafter called a 1.25 Gbps tributary slot and a 2.5 Gbps tributary slot). In the case of the 1.25 Gbps tributary slot, the numbers of tributary slots ts are defined as illustrated in FIG. 2: ts=2 for HO_ODU1, ts=8 for HO_ODU2, ts=32 for HO_ODU3, and ts=80 for HO_ODU4.
In the case of the 2.5 Gbps tributary slot, the numbers of tributary slots ts are defined as illustrated in FIG. 3: ts=4 for HO_ODU2 and ts=16 for HO_ODU3. In FIGS. 2 and 3, TS #i (i=1-80) indicates a tributary slot number, OH stands for overhead, FS stands for fixed stuff, and FEC stands for forward error correction. OH, FS, and FEC are also stored in tributary slots.
FIG. 4 illustrates exemplary mapping of an ODU0 frame and an ODU1 frame to an OPU2 frame. In the example of FIG. 4, the ODU0 frame is mapped to TS #1 in the payload area of the OPU2 frame and the ODU1 frame is mapped to TS #4 and TS #8 in the payload area of the OPU2 frame. In this case, the number of tributary slots M that the ODU1 occupies in the payload area of the HO_ODU2 frame is two.
An exemplary process of multiplexing ODUj frames in an HO_ODUk frame is described below.
(1) According to the combination of the ODUj frames, the HO_ODUk frame, and the tributary slot bit rate, one of the following two procedures is selected as a multiplexing/demultiplexing scheme: an asynchronous mapping procedure (AMP) and a generalized mapping procedure (GMT).
(2) The number of tributary slots M and the positions of the tributary slots occupied by each ODUj frame in the payload area (OPUk) of the HO_ODUk frame are determined according to the bit rate of the ODUj frame.
(3) The ODUj frame is placed in M tributary slots of the HO_ODUk frame while stuffing the HO_ODUk frame by inserting null data based on the difference between the sum of bit rates of the M tributary slots and the bit rate of the ODUj frame according to the AMP or the GMP.
The AMP and the GMP employ different frequency justification schemes. The GMP is a new method introduced when the ITU-T recommendation G.709 was revised in December 2009. In the AMP, multiplexing/demultiplexing is performed while absorbing the frequency difference and the frequency deviation between tributary slots of the ODUj frame and the HO_ODUk frame by stuffing the HO_ODUk frame in units of bytes (−1 through +2 bytes). In the GMP, multiplexing/demultiplexing is performed while absorbing the frequency difference and the frequency deviation between tributary slots of the ODUj frame and the HO_ODUk frame by stuffing the HO_ODUk frame in units of M bytes. Here, M corresponds to the number of tributary slots of the HO_ODUk frame that are occupied by the ODUj frame. Before the revision of the ITU-T recommendation G.709 (i.e., before December 2009), only the AMP was being used for multiplexing/demultiplexing of signal frames. Currently, multiplexing/demultiplexing of signal frames needs to be performed in an environment where both the AMP and the GMP are used.
In the AMP, as illustrated in FIG. 1, three justification control (JC) bytes and a negative justification opportunity (NJO) byte in the OPUk overhead and positive justification opportunity (PJO) bytes in the OPUk payload are used. Data or stuff bytes (zeros) are inserted in the NJO byte and the PUG bytes based on JC byte information (or frequency justification information) represented by the JC bytes. Thus, in the AMP, stuff positions where stuff bytes are inserted are fixed.
In GMP, stuff bytes are inserted in the OPUk payload according to JC byte information represented by six JC bytes in the OPUk overhead. Thus, in the GMP, stuff positions are changed according to the stuffing amount, and stuffing is performed based on the JC byte information in the previous (multi-)frame.
FIG. 5 is a drawing illustrating an exemplary configuration of an OTN_ADM (add drop multiplexer) cross connect apparatus. FIG. 6 is a drawing illustrating an exemplary configuration of an OTN_ADM multiplexing apparatus. In FIG. 5, an HO_OTUk optical signal input from an optical network to an HO interface 11/12 (i.e., one of the HO interfaces 11 and 12) is terminated by a demapping unit (OTUk DMAP) 13 of the HO interface 11/12 and an HO_ODUk signal is extracted. The HO_ODUk signal is separated (or demultiplexed) into HO_ODUj signals or LO_ODUj signals by a demultiplexer (OTUk DMUX) 14. The HO_ODUj signals are cross-connected by a cross connect unit 20 and supplied to a multiplexer (HO_ODUk MUX) 15 of the opposing HO interface 12/11 (i.e., the other one of the HO interfaces 11 and 12). Meanwhile, the LO_ODUj signals are cross-connected by the cross connect unit 20 and supplied to demapping units (LO_ODUj DMAP) 22 of an LO interface 21.
The demapping units (LO_ODUj DMAP) 22 of the LO interface 21 demap client signals from the LO_ODUj signals. The client signals are output to a client network via a client interface 23. Meanwhile, client signals input from the client network are received by a client interface 24 and mapped to LO_ODUj signals by mapping units (LO_ODUj MAP) 25. The LO_ODUj signals are cross-connected by the cross connect unit 20 and supplied to the multiplexer 15 of the HO interface 11/12.
The multiplexer 15 of the HO interface 11/12 multiplexes the supplied HO_ODUj signals and LO_ODUj signals and thereby maps them to an HO_ODUk signal. The HO_ODUk signal is mapped by an OTUk mapping unit (OTUk MAP) 16 to an OTUk signal and the OTUk signal is output to the optical network.
The OTN_ADM multiplexing apparatus of FIG. 6 has a configuration similar to that of the OTN_ADM cross connect apparatus of FIG. 5 except that the OTN_ADM multiplexing apparatus does not include the cross connect unit 20. In FIGS. 5 and 6, the same reference numbers are assigned to the corresponding components.
FIG. 7 illustrates intermediate frames and mapping types used to multiplex LO_ODUj signals in an HO_ODUk signal by the multiplexer 15. FIG. 8 illustrates intermediate frames and mapping types used to multiplex HO_ODUj signals in an HO_ODUk signal by the multiplexer 15. In the “intermediate frame” column, “ODTUG1” indicates ODTU group 1, payload type (PT)=20 indicates a frame where only the AMP is used for mapping, and PT=21 indicates a frame where either AMP or GMP can be used for mapping. As illustrated in FIGS. 7 and 8, different intermediate frames are used for different types of input signals. Therefore, in the related art, mapping and multiplexing circuits are provided separately for the respective types of input signals.
FIG. 9A illustrates a structure of an intermediate frame ODTUjk for the AMP where tributary slots in the OPUk payload area are multiplexed in units of bytes. When the bit rate of one tributary slot is 1.25 Gbps and jk=01, the ODTUjk overhead is 4×ts bytes. Also, when jk=01, the ODTUjk payload is 15232×ts bytes. The numbers of columns and rows of the ODTUjk payload and the value of is are defined by a table of FIG. 9B when the bit rate of one TS is 2.5 Gbps and defined by a table of FIG. 9C when the bit rate of one TS is 1.25 Gbps.
FIG. 10A illustrates a structure of an intermediate frame ODTUk.ts for the GMP where tributary slots in the OPUk payload area are multiplexed in units of bytes. When k=2, 3, or 4, the ODTUk.ts overhead is 6×ts bytes. Also, the ODTUk.ts payload is 15232×ts bytes when k=2 or 3 and is 15200×ts bytes when k=4. The numbers of columns and rows of the ODTUk.ts payload are defined by a table of FIG. 10B.
Meanwhile, Japanese Laid-Open Patent Publication No. 2004-523959, for example, discloses a method of transferring SDH/SONET/OTN frames via an intermediate network. In the disclosed method, contents of an entity are mapped to subframes, the subframes are virtually combined using sequence indicators assigned to the subframes and transferred via the intermediate network, and the subframes are assembled into the original entity at a remote node.
Also, WO2008/035769, for example, discloses an OTN multiplex transmission method that makes it possible to improve the multiplexing efficiency. In the disclosed OTN multiplex transmission method, management overheads are attached to CT signals, multiple CT signals whose bit rates are different from each other and are not integral multiples or divisions of each other are multiplexed, and some or all of the bit rates of the CT signals are adjusted such that the bit rates become integral, multiples or divisions of each other.
FIG. 11 illustrates an exemplary configuration of a related-art multiplexing and mapping unit that multiplexes four ODU2 frames in an ODU3 (OTU3) frame. In this configuration, four sets of a clock conversion buffer 31, an intermediate (ODTU23) framing unit 32, and a JC determination unit 33 are provided for the corresponding ODU2 frames, and the four sets operate independently of each other. After the four sets of the clock conversion buffer 31, the intermediate (ODTU23) framing unit 32, and the JC determination unit 33, a port switch 34, a slot switch 35, and an OTU3 framing unit 36 are provided.
FIG. 12 illustrates an exemplary configuration of a related-art multiplexing and mapping unit that multiplexes ODU2 and ODU1 frames in an ODU3 (OTU3) frame. With this configuration, up to four channels (CM) of ODU2 frames or up to 16 channels of ODU1 frames can be multiplexed in an ODU3 frame. To flexibly multiplex ODU2 and ODU1 frames, multiple sets of a buffer 31, an intermediate (ODTU13, ODTU23) framing unit 32, and a JC determination unit 33 are provided for the number of channels (or frames) in each of an ODTU23 block 37 and an ODTU13 block 38.
With the related-art configuration, however, when multiplexing, for example, one channel of the ODU2 frame and 12 channels of the ODU1 frames in an ODU3 frame, three sets of the buffer 31, the intermediate framing unit 32, and the JC determination unit 33 in the ODTU23 block 37 and four sets of the buffer 31, the intermediate framing unit 32, and the JC determination unit 33 in the ODTU13 block 38 are not used. Thus, the related-art configuration is redundant and unnecessarily increases the circuit size.
FIG. 13 illustrates an exemplary configuration of a related-art multiplexing and mapping unit that multiplexes ODU2, ODU1, and ODU0 frames in an ODU3 (OTU3) frame. With this configuration, up to four channels of ODU2 frames, up to 16 channels of ODU1 frames, or up to 32 channels of ODU0 frames can be multiplexed in an ODU3 frame. To flexibly multiplex ODU2, ODU1, and ODU0 frames, multiple sets of a buffer 31, an intermediate (ODTU03, ODTU13, ODTU23) framing unit 32, and a JC determination unit 33 are provided for the number of channels (or frames) in each of an ODTU23 block 37, an ODTU13 block 38, and an ODTU03 block 39. Similarly to the configuration of FIG. 12, not all of the sets of the buffer 31, the intermediate framing unit 32, and the JC determination unit 33 in the ODTU23 block 37, the ODTU13 block 38, and the ODTU03 block 39 are always used. Accordingly, the configuration of FIG. 13 is also redundant and unnecessarily increases the circuit size.
As described above, with related-art configurations, the circuit size (or the number of circuits) of a multiplexing unit (and/or a mapping unit) drastically increases as the types of input ODUj frames increase. In ITU-T G.709, as illustrated in FIGS. 7 and 8, multiplexing of various types of ODUj frames is standardized. Therefore, an GIN transmission apparatus needs to support multiplexing of various types of ODUj frames. However, this may increase the circuit size of the GIN transmission apparatus.